Method of making sheathed electric heating units



Nov. 7, 1961 c. H. YOHE 3,007,235

METHOD OF MAKING SHEATHED ELECTRIC HEATING UNITS Filed Sept. 2, 1958INVENTOR. CHARLES H. YOHE Q LMA/ ATTORNEY 3,007,235 METHOD OF MAGSI-EATHED ELECTRIC HEATING UNITS Charles H. Yohe, Glen Ellyn, llll.,assignor to General Electric Company, a corporation of New York FiledSept. 2, 1958, Ser. No. 758,241 3 Claims. (Cl. 29-15564) The presentinvention relates to methods of making sheathed electric heating units,and, more particularly, to a method for making a plurality of such unitsfrom a single assembly.

An electric heating unit of the sheathed type usually comprises anelectric resistance conductor or element having a helical form, anenclosing tubular metal sheath, and a body of compressibleheat-conducting and electrical-insulating material embedding theresistance conductor and holding it in a substantially central locationwithin the sheath. One of the methods presently employed inmanufacturing such electric heating units is disclosed in U.S. PatentNo. 2,677,172, granted on May 4, 1954, to Sterling A. Oakley. In thatpatent, the method disclosed is that of compacting and compressing theheat-conducting and electrical-insulating material around and throughoutthe electric resistance conductor by means of cold-rolling the assemblythrough a series of angularly off-set rolls which have grooves designedto grip and compress the metallic sheath around the enclosed materials.According to that disclosed method of making sheathed electric heatingunits, the unit is produced by winding the resistance conductor on amandrel to produce a helix, providing the helix with electricalterminals, centrally locating the helix within a tubular sheath, closingoff the bottom end of the sheath with a combustible washer, and thenfilling the sheath in an upright position with a heat-conducting andelectricalinsulating material such as magnesium oxide in a granulatedform. The insulating material, which is tamped down firmly during theloading operation, fills the space between the helix and the walls ofthe tubular sheath and also fills up the void in the interior of thehelix. Subsequently, the assembly is subjected to a plurality ofsuccessive, gradual, cold-rolling passes which act to compress andelongate the sheath and to compact the insulating material in and aroundthe centrally located helix.

it can readily be seen that each electric heating unit which is producedby this method has to be produced as an individual unit. Correctallowance must be made when cutting the helical winding and the tubularsheath to length prior to the subsequent rolling and elongatingoperations, in order that the finished electric heating unit will be ofproper length. Likewise the necessity of fitting the helix with theelectrical terminals prior to insertation of the helix into the sheathand subsequent cold-rolling necessitates that the terminals be afiixedby resistance welding or some other method to securely fasten theterminals to the helix. Such a method of ailixing the terminals to thehelix is disclosed in US. Patent No. 2,546,315 granted on March 27,1951, to S. A. Oakley. As pointed out in that patent, one of the primaryconsiderations which led to the introduction of resistance welding theterminals to the resistance conductor helix was the tendency of theconductor to pull away from the terminal during the operations ofloading the sheath with the insulating material and the subsequentoperation of elongating the sheath to comp-act the insulator. Thistendency is accentuated where the helix is composed of a fine wireresistance conductor.

These and other problems led the applicant to strive to improve themanufacturing efliciency of producing such sheathed electric heatingunits by fabricating a single elongated sheathed electric heatingassembly and producice Patented Nov. 7, 1961 ing a plurality of electricheating units of the proper length by severing them from the assembly.In order to accomplish the foregoing, a suitable method of attaching theterminals to the severed portions had to be devised, since it would nolonger be possible to attach terminals to the resistance conductor priorto the loading and cold-rolling operations. Likewise, the presence ofthe compacted insulating material inside the helix made any method ofterminal attachment impracticable, since the highly compacted insulatingmaterial had to be gouged out from the interior of the helix for asubstantial distance in order to expose a sufficient length of theresistance wire helix to insure electrical connection with the terminal.Because of the fineness of the resistance wire and small inside diameterof the helix, it would be exceedingly difficult to avoid damage to thehelix.

Accordingly, it is an object of the present invention to provide amethod for manufacturing a plurality of terminal-fitted, sheathedelectric heating uni-ts from a single elongated assembly.

Another object of the invention is to provide an improved method formanufacturing such an elongated assembly which will be free to receivean electricterminal at any point of severance along the winding.

The invention, in one form, is practiced by winding the resistanceconductor helix in such a manner that the turns of the helix are inturn-to-turn relationship to one another so as to preclude the entranceof any substantial quantity of the heat-conducting andelectrical-insulating material into the void in the interior of thehelix during the loading operation. The insulating material is tampedbetween the interior walls of the tubular sheath and the exterior of thehelix and is subjected to a series of coldrolling passes which compactthe insulating material densely around the exterior of the helix. Due tothe manner of winding the helix, not more than a very small quantity ofthe insulating material will enter into the interior of the helix duringthese cold-rolling operations. Thus when the desired length of electricheating unit is severed from the elongated assembly, the interior of thehelix will be free to receive a terminal pin as will be described.

The subject matter which I regard as my invention is particularlypointed out and distinctly claimed in the concluding portion of thisspecification. My invention, however, both as to organization and methodof practice, together with further objects and advantages thereof, maybest be understood by reference to the following description taken inconnection with the accompanying drawing in which:

FIG. 1 is a side elevational view of the loaded sheath in its assembledstate subsequent to cold-rolling and annealing.

FIG. 2 is a fragmentary perspective view of the assembly following theloading operation, and undergoing coldrolling.

FIG. 3 is an enlarged transverse sectional view taken along the line 33in FIG. 2, looking in the direction of the arrows.

FIG. 4 is a view taken along the line 44 in FIG. 2, looking in thedirection of the arrows.

FIG. 5 is a view taken along the line 5-5 in FIG. 2, looking in thedirection of the arrows.

FIG. 6 is a view taken along the line 6-6 in FIG. 2, looking in thedirection of the arrows.

FIG. 7 is a side elevational view, partly in section, of an electricheating unit, as severed from the elongated assembly.

Referring now to the drawing, in FIG. 1 there is illustrated theassembly as it appears subsequent to the coldrolling operation. Theassembly may be, for example, of a length of 10 or 12 feet. The finalelectric heating units which will be produced from this assembly, may,on the other hand, be only a few inches long. The assembly, which isshown generally at 1, comprises an outer metallic tubular sheath 2 thatmay be formed of a suitable nickel-chromium-iron alloy which has asubstantially circular cross-section, and an electrical resistanceconductor or element 3 that may be formed of a suitable nickelchromiumalloy and located substantially centrally within the sheath 2 andsurrounded by a body of heat-conducting and electrical-insulatingmaterial 4 such as magnesium oxide. The insulating material 4- is infinely divided or granulated form and holds the resistance conductor 3in a substantially coaxial relationship with the sheath. The resistanceconductor 3 comprises a helix which is formed by winding the resistancewire on a suitable mandrel to establish the ultimately desiredresistance per linear inch of helix. Pursuant to this invention, thehelix is initially wound in tight turn-to-turn relationship and is ofsuch length relative to the sheath that when it is inserted into thesheath and is stretched as a result of the elongation of the sheath aspresently described, the resulting spacing between the turns will not besufficient to admit any substantial quantity of the insulation materialinto the interior of the helix during any of the manufacturingoperations. In the preliminary assembly In, FIG. 2, the helix is fittedwith terminal members 5, preferably pursuant to the above-noted Patent2,546,315. The terminai members act both to close off the ends of thehelix to prevent the entrance of the insulating material therethroughand to provide a support to hold the helix coaxial with the sheathduring the loading operation. A combustible washer (not shown) isprovided at the lower end of the assembly in, pursuant to Oakley et al.US. Patent 2,483,839, granted October 4, 1949, and the insulatingmaterial is then loaded thereinto while it is in an upright position ina suitable loading machine such as is shown in US. Patent 2,316,659,granted on April 13, 1943, to l. L. Andrews. The material is tamped downfirmly during the loading operations so that the insulating materiallies between the interior walls of the tubular sheath and the exteriorof the helix.

The powdered magnesium oxide insulating material is usually a mixture ofgrains of several sizes, as follows:

The remaining four percent will pass through the final screen. It isprobable that at least some of this latter insulating material will alsopass between the turns of the helix and accumulate at the bottom of thehelix during the loading operation because of the vibration orreciprocation of certain centering devices employed in the said Andrewsloading machine. However, the quantity is so small that it can have noeffect on the later operations.

After the loading has taken place, another combustible washer (notshown) is provided at the top of the elongated winding and the assemblyis then subjected to a series of cold-rolling operations which aresubstantially as disclosed in the aforementioned U.S. Patent No. 2,677,-172. In brief, according to the method described in the patent, a seriesof matching rolls are arranged in vertical sequence. Individual drivesare provided for rotating each succeeding set of rolls, which lies overthe adjacent set of rolls, at an increased rate of speed in order toadjust for the elongation of the assembly as it is passed between thesets of rolls. Such an arrangement may be seen by referring to FIG. 2which shows the assembly 1a in vertical position passing through thefirst three sets of rolls, numbered 8a, 8b, 9a, 917, 10a, and 1%.

As the assembly commences the cold-rolling process, a cross sectionthrough the winding, seen in FIG. 3, illustrates the absence ofinsulating material inside the helix. Each set of rolls, for example,the set of rolls shown as 8a and 81) have complementary grooves 11a and11b which serve to provide a gripping action and to propel the windingupward through the rolls. The grooves are, in the initial stages,semi-elliptical in cross-section, in order to deform the tubular sheathinwards around the helix and thus to compress and compact the insulatingmaterial, as may be seen in FIG. 4.

The succeeding set of rolls 9a and 9b, which are disposed adjacent andvertically above rolls 8a and 8b, are positioned at right angles to thelower rolls, in order to compress the tubular sheath so as to distort itat right angles to the previous distortion. Also the ellipse formed bythe grooves 112a and 12b in the set of rolls shown as 9:: and 9b is ofsmaller cross-section than the preceding elliptical cross-section. Thearrangement of the view looking downward on rolls 9a and 912 may be seenin FIG. 5.

Subsequent passes, the next of which may be seen in FIG. 6, are throughmatching sets of rolls forming ellipses of decreasing cross-sections,each set of rolls running at a greater speed than the preceding set. Thefinal rolls (not shown) have grooves which form circular crosssectionsin order to produce a final, densely compacted assembly having acircular cross-section. As may be seen in PlGS. 3, 4, 5, and 6, theinsulating material is compressed between the exterior of the helix andthe inner walls of the tubular sheath. During the rolling operation, thehelix is stretched consistent with the elongation of the sheath, andalthough the individual turns of the helix will then be separated theone from the other as shown in FIG. 1, only a small amount, if any, ofthe insulating material enters the interior of the helix, by virtue ofthe initially closely adjacent turns of the helix which were providedfor this purpose. At the conclusion of the coldrolling operation, theinsulating material about the outside of the helix is quite hard anddense. The elongated winding is next subjected to an annealing operationto remove the hardening effects of cold-rolling; during the annealing,the combustible washers burn themselves out.

At this stage of the operation, represented by FIG. 1, the elongatedassembly is ready to be divided into shorter lengths or segments formaking electric heating units. At FIG. 7 may be seen such a segmentwhich, in practice, may be cut out of any portion of the elongatedassembly such as, for example, through the two points indicated at 77 inFIG. 1. It is to be understood that such severing occurs after thecold-rolling and annealing have been completed.

Reference to FIG. 7 shows the segment, which has een cut out of theelongated winding, during two stages of affixing the terminals to theends of the segment. At the lower end of the drawing is shown one end ofthe segment after a reaming operation has been performed in order toremove the insulating material from between the sheath and the helix andto clip off a. portion of the helix so as to produce a void space 13which will subsequently receive a final heat-resistant seal of somematerial such as glass or plastic. An electrical terminal pin 14 havingexternal threads adapting it to be screwed into the helix, is shown inposition for insertion into the end of the helix. The pin is therebyaffixed to the helix on the interior surface thereof in intimaterelation so as to provide both a positive mechanical and a goodelectrical connection. Alternatively, the connection may be accomplishedby force fit, such as by driving a slightly oversize, smoothsurfacedterminal pin, such as the upper terminal pin 14a, FIG. 7, into thehelix. Both of these terminal pin constructions are common in the art.

This is obviously a much more satisfactory operation than would berequired if it were necessary to remove compacted magnesium oxide fromthe interior of the helix to accommodate the rather substantialpenetration of the terminal pin thereinto. In addition to the inherentdifiiculty of removing the compacted material from a central area whichmay be only 0.09" in diameter, it is obviously necessary that theresistance wire itself-which may be #27 gauge (0.0142" in diarneter)beundamaged during the process.

The top portion of FIG. 7 shows an electric heating unit as it appearsin final form with the terminal pin inserted in the helix and the finalterminal seal 15 applied. The terminal seal 15 may be applied in themanner described in Patent No. 1,992,787, granted on February 26, 1935,to Robert I. Sutton. Depending on the ultimate use of the heatingelement, the seal may also be of the various plastic materials which arenow available for such use.

Thus it can be seen that the invention, as disclosed, provides -a methodfor producing a plurality of segments which are easily adapted toreceive terminal connections. Whereas previous methods compacted theinsulating material both inside and outside of the helix, in the presentmethod, the interior of the helix is clear to receive a terminal pinwhich can make a good mechanical and electrical connection. There is nonecessity for reaming out the interior of the helix and possiblydamaging the resistance conductor winding. The terminal pin may beattached by a simple mechanical connection rather than by rested-ing toresistance welding.

Also it can be seen that by producing an elongated winding, substantialsavings can be effected in the manufacturing operation by reducinghandling costs, since the winding is produced as a single unit duringloading, rolling, and annealing. Previous methods necessitated thehandling of many smaller units and entailed greater expense than thedisclosed method due to the cost of handling these small units.

An additional saving is accomplished since no allowance need be made.for the elongation of the element in passing through the series ofrolls. As previously mentioned, such allowance had .to be carefullycalculated in order to produce a resultant unit of the proper length.Some waste was therefore encountered with units which sometimes did notachieve the proper length. In the disclosed method, little regard needbe paid to the resultant length of the elongated assembly, since thesegments are cut from this winding to the proper length, after thecold-rolling has taken place.

Another advantage of the invention results from the fact that fora givenunit of length of the finished heating unit, the closer spacing of theturns of the resistance helix requires a greater overall length ofresistance wire as compared with that of presently known units. For thesame resistance per unit of length, therefore, the resistance wire ofthe present invention will be of heavier gauge, resulting in a lowerwire temperature during use, and longer life.

While there has been described what is at present considered to be thepreferred embodiment of the invention, it will be understood thatvarious modifications may be made therein, and it is intended to coverin the appended claims all such modifications which fall within thescope of this invention.

I claim:

1. The method of making an elongated heating unit assembly suitable fordivision into shorter lengths to which electrical connection terminalsmay be applied, which includes the steps of providing a quantity offinely divided granular insulation material in which the majority of thegrains exceed a certain size, providing a resistance wire helix in whichthe spacing between each turn and its next succeeding turn is less thanthe size of the said majority of insulation material grains, confiningsaid helix axially within a straight length of tubular metallic sheathin spaced relation to the inner wall thereof, filling only the spacebetween said helix and the said sheath with said insulation material,confining the insulation material against escape from said sheath, andreducing the diameter of said sheath along the length thereof tocomp-act the insulation material into -a hard, dense mass between thehelix and said sheath while maintaining the spacing between the turns ofthe helix too small to pass the said majority of the granular materialand thereby keeping interior of said helix substantially free ofinsulation material, whereby at any intermediate length severed from theassembly, the said helix will freely admit terminal members interiorlythereof in electrical connection therewith.

2. The method of making an elongated heating unit assembly suitable fordivision into shorter lengths to which electrical connection terminalsmay be applied,

which includes the steps of providing a quantity of finely dividedgranular insulation material in which the majority of the grains exceeda certain size, providing a resistance wire helix in which each turn andits next succeeding. turn are in tight turn-to-turn relation, confiningsaid helix axially within a straight length of tubular metallic sheathin spaced relation to the inner wall thereof, filling only the spacebetween said helix and the said sheath with said insulation material,confining the insulation material against escape from said sheath, andelongating the sheath and therein contained helix to reduce the diameterof said sheath to compact the insulation material into a hard, densemass between the helix and said sheath while establishing a spacingbetween the turns of the helix insufiicient to pass the said majority ofthe granular insulation material therethrough, whereby the said helixwill be free of compacted insulation material and thereby accommodateterminal members interiorly thereof in electrical connection therewith.

3. The method of making a plurality of terminal-fitted electric heatingunits from a single, elongated assembly, which includes the steps ofproviding an electrical resistance conductor wound in the form of ahelix having a multiplicity of turns of substantially uniform diameterin which each turn is closely adjacent the next succeeding turn,securing a conductive terminal within each end of the helix inelectrical connection therewith, disposing said helix axially within astraight length of tubular metallic sheath in spaced relation to theinner wall thereof with the said terminals projecting axially therefrom,filling the space between said helix and said wall within finely dividedgranular insulation material in which the majority of the grains are toolarge to pass into the center space of the helix, confining saidinsulation material against escape from said sheath, compacting saidinsulation material in said space into a hard, dense, mass whilemaintaining the spacing of the coils of the helix sufficient to keep theinterior of the helix substantially free of insulation material,dividing the thus-prepared assembly into desired shorter lengths, andinserting conductive terminal members into the open ends of the helix asneeded to provide each of the shorter lengths with an electricalterminal at each end thereof.

References Cited in the file of this patent UNITED STATES PATENTS1,523,434 Lightfoot et al. Jan. 20, 1925 2,093,810 Karmazin Sept. 21,1937 2,166,109 Karmazin July 18, 1939 2,546,315 Oakley .Mar. 27, 19512,629,922 Finch Mar. 3, 1953 2,677,172 Oakley May 4, 1954

